Flow controllers are typically configured to deliver a fluid based on set points that are in terms of fluid flow (e.g., cubic centimeters per minute or grams per minute). When a set point is received, a flow controller adjusts, for example, a valve to deliver the fluid at the specified fluid flow set point in a steady-state mode. The flow controller generally adjusts the position of the valve to control the flow of the fluid around the set point using a control algorithm that is based on, for example, proportion-integral-derivative (PID) control.
Known flow controllers can be programmed to deliver fluid by receiving, for example, a pulse width and a set point much like a fuel injector. But, unexpected deviations in fluid flow away from the set point as a consequence of, for example, pressure changes can result in a significant deviation in the final amount of fluid delivered relative to the set point. Present flow controllers do not have the ability, based on feedback control, to change/adjust set points and/or delivery times (e.g., delivery time window) to accurately deliver a specified-quantity of a fluid. Additionally, known flow controllers can correct for deviations away from a steady-state fluid flow set point, but are not configured to correct for deviations away from a quantity set point.
Present flow controllers are functional, but are not sufficiently accurate or otherwise satisfactory if used to deliver a specified-quantity of a fluid. Accordingly, there is a need for a method and apparatus for receiving and delivering, based on feedback control, a specified-quantity of a fluid in response to receiving a quantity set point.